1. Field of the Invention
Present invention relates generally to circuit boards used in information handling systems. More specifically, the present invention provides an improved method and apparatus for increasing the routing density for circuit boards comprising differential conductor pairs.
2. Description of the Related Art
As the value and use of information continues to increase, individuals and businesses seek additional ways to process and store information. One option available to users is information handling systems. An information handling system generally processes, compiles, stores, and/or communicates information or data for business, personal, or other purposes thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems.
As computers have increased in performance, it has become necessary to use higher data bit rates in high-density circuitry with minimal spacing between the various circuit conductors. As a result, it has become difficult to fabricate circuit boards that meet the needs of high performance computing technology using conventional circuit board manufacturing methods. The necessity for a more ideal transmission path vital to higher-performance electrical signals is in direct conflict with the minimization of both cost and size.
One of the most common trends for achieving higher performance is the use of Low Voltage Differential Signaling (LVDS) to transmit data both intra-box and interbox. LVDS uses a pair of signals of opposite polarity to transmit data. There are currently many busses that use LVDS to transmit data in the Gigahertz range. These busses require very fast edge rates to support the signal integrity of the digital waveforms they produce. As these become increasingly faster, crosstalk between differential pairs becomes a greater concern. Faster edge rates result in an environment where very short parallel runs of conductors can produce significant crosstalk, thus increasing the risk of corrupted data. Increasing the spatial isolation of signals to minimize crosstalk translates to a reduction in routing density. There is a need, therefore, for a method and apparatus to route these signals to achieve acceptable routing density while minimizing or eliminating crosstalk.
In view of the shortcomings of the prior art, there is a need for an improved method and apparatus for increasing the density of circuits on printed circuit boards, particularly those employing mixed-mode signal conductors and differential conductor pairs.
In accordance with the present invention, a method and apparatus is provided which substantially decreases crosstalk between conductors, particularly differential conductor pairs, that are routed in high-density patterns on printed circuit boards. In one embodiment of the invention, a single-ended conductor is divided at a first point into two single-ended conductors of equal width, with the two single-ended conductors being routed around and alongside a differential pair of conductors. In this embodiment, equal and opposite noise is coupled onto each branch of the single-ended signal from each side of the differential pair. The two single-ended conductors are rejoined at a second point to form a combined single-ended conductor. Signals traveling along the two separate single-ended conductor paths are combined at the second point and noise carried in the respective signals on the two paths is cancelled when the two paths are combined. Noise coupled into the differential pair from the two single-ended conductors is eliminated at the receiving end as common mode noise.
In other embodiments of the present invention, the single-ended conductors and the differential pairs can be oriented between two planes in stripline or dual stripline configurations. Furthermore, the present invention can be implemented by utilizing a single microstrip layer or a combination of a microstrip with buried microstrip. The method and apparatus of the present invention can be implemented in numerous other media, such as vias and cables (including flex and ribbon cables).
Utilizing the method and apparatus of the present invention, it is possible to achieve significantly greater conductor densities in circuit boards or similar structures because the noise rejection characteristics make it possible to take advantage of space that is wasted in prior art methods for routing single-ended conductors and differential conductor pairs where noise is reduced primarily by spatial isolation.